1. Field of the Invention
The present invention relates generally to monolithic microwave integrated circuits.
2. Description of the Related Art
The dual developments of compound semiconductors and heterostructure transistors have facilitated the improved frequency performance of microwave and millimeter-wave circuits. For example, superior high-frequency performance has been achieved by forming each transistor region of heterojunction bipolar transistors (HBTs) with semiconductors of selected bandgaps. In an exemplary HBT, the emitter semiconductor is chosen to have a greater bandgap than the base semiconductor. This allows the emitter to be heavily doped for increased emitter efficiency and the base to be heavily doped and narrowed without increasing base resistance. In addition, the collector semiconductor can be chosen to increase the transistor's breakdown voltage.
Previously, realization of the advantages of heterostructure transistors was primarily limited to the gallium aluminum arsenide/gallium arsenide (GaAlAs/GaAs) and indium gallium arsenide/indium phosphide (InGaAs/InP) semiconductor systems. The advantages of the GaAlAs/GaAs system include an excellent lattice match, well-developed growth technologies (e.g., molecular beam epitaxy and metal-organic chemical vapor deposition), availability of large bandgap differences, high electron mobility, and a semi-insulating GaAs substrate. GaAlAs/GaAs HBTs have achieved unit current gain cutoff frequencies (f.sub.T) of 100GHz.
The advantages of the InGaAs/InP system include a good lattice match, the same well-developed growth technologies of the GaAlAs/GaAs system, an electron mobility even higher than GaAs, availability of even larger bandgaps, and a semi-insulating substrate of InP which has a good thermal conductivity. HBTs in the InGaAs/InP system have achieved f.sub.T values of 165 GHz.
Conduction of microwave signals in these semiconductor systems is typically accomplished with microwave transmission structures such as microstrip. The dielectric of these structures preferably has a low dielectric constant and a low loss tangent to reduce parasitic capacitances and dielectric losses. Typically, the substrate of these semiconductor systems has formed the dielectric of the microwave transmission lines. Substrates in the GaAlAs/GaAs and InGaAs/InP systems can be fabricated with high resistivities (e.g., 10.sup.8 .OMEGA.-cm) through the addition of carrier-trapping impurities such as chrome or iron. Accordingly, these substrates can be used to form low-loss transmission lines.
Recently, another semiconductor system has shown promise in the microwave and millimeter-wave regions. This is the SiGe/Si system. Ge.sub.x Si.sub.1-X is an alloy with a lattice constant that is mismatched from Si. The SiGe/Si system has been shown to have excellent high-frequency performance. For example, Si/SiGe HBTs have achieved f.sub.T values of 70 GHz. In addition, silicon has far and away the most mature semiconductor technology and enjoys a significant cost advantage over other semiconductor systems.
However, in contrast with the GaAlAs/GaAs and InGaAs/InP systems, the resistivity of silicon is limited (e.g., to .about.10.sup.4 .OMEGA.-cm) because the resistivity of silicon can only be increased by purification. Consequently, microwave transmission lines fabricated over silicon substrates produce undesirable losses. Primarily for this reason, MMICs constructed in the low-cost SiGe/Si system have not exhibited competitive performance.
Sakai, Hiroyuki, et al. have proposed (Sakai, H. et al., "A novel millimeter-wave IC on Si Substrate", 1994 IEEE MTT-S Digest, pp. 1763-1766) a microstrip transmission line whose ground plane is fabricated on the surface of a highly-doped, silicon substrate. In this structure, millimeter-wave heterojunction transistor chips are inverted and bonded to signal lines of the transmission line. Although this technique realizes microwave circuits with a low-cost silicon substrate, it fails to take advantage of the high yields and inherent cost savings of monolithic circuit fabrication.